The present invention relates to a process for the fractionation and purification of aromatic polyamine mixtures and the use of the mixtures. Numerous patent applications and patents describe the preparation of aromatic polyamines and polyamine mixtures, in particular of the diphenylmethane series, and also the use of these products. These publications place great importance on the use of these products as raw materials for manufacturing isocyanates, generally by reacting the polyamine mixtures with phosgene by the known methods generally practiced.
However, in many cases the resulting isocyanates or isocyanate mixtures arise in forms and compositions not appropriate to their preferred further use in the isocyanate stage, which must first be converted into the appropriate usable form by, in some cases costly, working-up and separation processes. Suitable precursor polyamines which can be converted at less expense into the isocyanate forms to be used frequently give rise to process engineering problems, are completely inaccessible or are economically unattractive to manufacture.
An example is 4,4'-diisocyanatodiphenylmethane, which is important in the manufacture of high-grade polyurethane materials, the precursor amine of which is generally obtainable from aniline and formaldehyde only jointly with isomers, in particular the 2,4'-isomer, and higher-functional polyamines. Although these constituents are the foundation for isocyanates which are equally in demand, the raw isocyanates are not readily separated into those isocyanates or isocyanate mixtures which are suitable for further use. Generally, this involves first separating some of the binuclear compounds from the remainder. The 4,4'-diisocyanatodiphenylmethane is then liberated from the other isomers from the binuclear fraction in a second distillation step which requires many separation stages.
The 2,4'-isomer in enriched form has recently itself acquired increasing importance as a raw material for polyurethane and it is only with considerable distillation effort that it can be enriched by comparison with the 4,4'-isomer and liberated from the 2,2'-isomer which is optionally present.
Isomer separation processes or enrichment processes within the fraction comprising higher-nuclear homologues or higher-functional constituents of amines and of isocyanates of the diphenylmethane series are virtually unknown.
4,4'-diaminodiphenylmethane is also gaining ground increasingly as a raw material for di-(4-isocyanatocyclohexyl) methane, the nucleus-hydrogenated form of 4,4'-diisocyanatodiphenylmethane, wherein it is very costly to prepare suitable aromatic polyamine mixtures for the hydrogenation stage having as high a 4,4'-diaminodiphenylmethane content as possible with simultaneously as low a proportion of 2,4'-diaminodiphenylmethane as possible.
It is known that amines can in some cases be separated by partially converting them into their salts, wherein, inter alia, use is made of the different base strengths. These are generally monoamines having widely differing base strengths. Such disproportionation effects have also already been described in two-phase systems in respect of aromatic polyamine mixtures, in particular of the diphenylmethane series (German Auslegeschriften 2,238,319 and 2,528,694). As a result of the numerous components present in such a mixture, whose amino groups differ hardly at all in terms of type--virtually all are arylamino groups--the effects are not particularly sizeable or pronounced in terms of being of interest for direct use with simple means.
The object was to provide a process which enables aromatic polyamine mixtures to be fractionated and/or purified in simple manner such that isomers arise in a pure or an enriched form.